1. Field of the Invention
This invention relates to the fabrication of printed circuit boards. More particularly, this invention relates to the fabrication of printed circuit boards by the pattern plating method in which a pattern plated metal, e.g., solder, is used as an etch resist. Still more particularly, this invention relates to post-etch cleaning processes for the fabrication of printed circuit boards by the pattern plating process.
2. Description of Related Art
The manufacture of printed circuit boards and the soldering of components therewith is generally disclosed in "Printed Circuits Handbook", Third Edition, edited by Clyde F. Coombs, Jr., McGraw-Hill, 1988. In particular, Coombs describes the pattern plating method (also known as the plate-and-etch method) in Chapter 11, pages 11.21-11.24. In the pattern plating method of circuit board manufacture, a polymeric or resin resist image is first formed on a copper clad circuit board substrate and then a metal which is resistant to etchants, e.g., a tin-lead solder alloy, is plated on the copper surface areas not protected by the polymer resist image to form a complimentary metal resist image. The metal resist material may be deposited electrolytically, e.g., by use of the tin or tin alloy plating baths as disclosed in Section 14.2.5, p.p. 14.4 et seq., of "Printed Circuits Handbook", supra; Bokisa et al., U.S. Pat. No. 4,885,064; Opaskar et al., U.S. Pat. No. 4,440,608; Davis U.S. Pat. 4,053,372; and Lerner et al., U.S. Pat. No. 4,033,835. Alternatively, the metal resist material may be deposited by an immersion plating process, e.g., such as the tin immersion plating processes disclosed in Holtzman et al., U.S. Pat. No. 4,657,632, wherein thiourea is used as a complexing agent, and Dodd et al., U.S. Pat. No. 5,196,053, wherein an imidazole-2-thione compound is used as a complexing agent. After pattern plating, the polymer resist image is then stripped from the copper surface and the uncovered copper not protected by the metal, e.g., by the tin-lead resist image, is removed from the substrate by an etchant to form the printed circuit. In subsequent, conventional soldering operations, a solder mask is first applied to the printed circuit board to cover all board areas except where components are to be soldered thereto. Prior to the application of the solder mask the electrolytically deposited tin-lead etch resist on the circuit typically is reflowed at elevated temperatures and since the reflow is not always uniform the circuit board sometimes has to be subjected to a leveling process. Such a leveling process comprises immersing the board in molten solder followed by passing the board over a hot air knife, i.e., a constricted elongated hot air jet, thereby to even the solder thickness and clear board through-holes of residual solder.
Conventional soldering operations are described in Part 5 of "Printed Circuits Handbook", supra. In particular, problems in reflowing plated coatings are identified in Section 23.7.3, p. 23.13, wherein it is recognized that codeposited impurities, especially copper in tin-lead, can cause dewetting in a reflowed deposit and that heavy oxidation or tarnish films that result from chemical attack by etching must be removed prior to reflowing. The causes of poor solderability are further identified in Section 23.11, pp. 23.16-23.17, and the use of precleaners are disclosed to restore solderability. In particular, it is recognized that tarnishing of the tin or tin-lead coating during etching or storage can detract from solderability. Typical acid cleaners disclosed for removing tarnish from tin or tin-lead contain thiourea, fluoboric acid, wetting and complexing agents.
Typical commercial post-etch, precleaners used to restore solderability such as described supra, use thiourea. However, since thiourea is a toxic material and has been identified as a carcinogen, additional measures are needed in handling such precleaners safely and disposing of process wastes while rendering them innocuous to the environment. There is a need for a high efficiency, post-etch cleaning process and precleaners therefor, which is safer to handle and is innocuous to the environment.